Aqueous process for making fluoropolymers

ABSTRACT

A novel aqueous polymerization process for making fluoropolymers is disclosed in which non-ionic non-fluorinated emulsifier is used to produce fluoropolymer emulsions. The emulsifiers used in the invention are those that contain segments of polyethylene glycol and/or polypropylene glycol with repeating units of 3 to 100.

This application is a Divisional application of, and claims priority under 35 U.S.C. §120 to, copending U.S. application Ser. No. 11/149,797 filed Jun. 10, 2005.

FIELD OF THE INVENTION

The invention relates to a process for making fluoropolymers using non-fluorinated, non-ionic emulsifiers. The emulsifiers contain segments of polyethylene glycol and/or polypropylene glycol.

BACKGROUND OF THE INVENTION

Fluoropolymers are generally made by an aqueous dispersion process, which provides a suitable heat sink for controlling the heat of polymerization and can produce a high yield and high molecular weight relative to polymerization conducted in an organic solvent. In order to achieve stable dispersion or emulsion, a suitable surfactant or emulsifier must be employed. Fluorinated-surfactants are generally used because they can yield stable particle and high molecular weight fluoropolymers. However, the fluorinated-surfactants typically used in emulsion polymerization of fluoropolymers, such as the ammonium salt of perfluoro octanoic acid or salts of perfluoro sulfonic acids are expensive. They also present an environmental concern related to bio-persistence.

It is therefore desirable to carry out an emulsion polymerization of fluoropolymers in the absence of fluorinated-surfactants, without compromising the properties of the resultant fluoropolymers. It would also be desirable to produce small particle size emulsion so that latex stability during storage as well as quality of film formation is improved. Furthermore, it would be desirable to produce a latex and/or fluoropolymer having fewer extractable ions and extractable low molecular weight polymers, while generally yielding fluoropolymers that have similar or even improved properties compared to analogous fluoropolymers made in presence of added fluorinated-surfactants.

Emulsion polymerization of vinylidene fluoride at moderate pressures and temperatures using fluorinated surfactants, free radical initiators, and trichlorofluoromethane as chain transfer agent is taught in the U.S. Pat. No. 4,569,978 in which VF2 based polymers are produced with reduced tendency to generate cavity and greater resistance to discoloration at elevated temperatures. The process was refined in the U.S. Pat. No. 6,734,264 wherein particularly ozone depleting agent (trichlorofluoromethane) was replace by propane which is environmentally friendly chemical. It is noteworthy that in both processes fluorinated surfactant was needed to produce stable emulsion. For example, perfluorocarboxylate salts was used to stabilize fluoropolymer emulsion polymerizations, with the most common example being ammonium perfluorooctanoate or ammonium perfluoronanoate. The high degree of fluorination is thought to be necessary to prevent chain transfer reaction between a growing polymer chain and the surfactant which in turn may result in lowering molecular weight and/or inhibition of the polymerization.

Many attempts have been made to find a suitable emulsifier in place of fluorinated surfactant for such polymerizations, as disclosed in the background section of U.S. Pat. No. 6,512,063 in which sodium salt of hydrocarbon sulfonates as non-fluorinated but ionic emulsifier was employed. The ionic emulsifiers are undesired for high purity applications due to high levels extractable ions. Furthermore, hydrocarbon sulfonates act as implicit chain transfer agent in the emulsion polymerization of fluoropolymers, as a result, it cannot be used in sufficient amount to produce small particle size latex without inhibiting such polymerizations.

Emulsifier-free aqueous emulsion polymerization process for making fluoropolymer such as TFE and/or VDF copolymers is described in U.S. Pat. No. 6,693,152. In emulsifier free emulsion polymerization, first only inorganic ionic initiators such as persulfates or permangamates may work whereas organic peroxide initiators would not work. Second, the particle size of emulsifier free emulsion of fluoropolymers would be large; as a result, the shelf-life of latex would be very limited. Third, the solid content of emulsifier free latex is limited to low or moderate solids, where in fact a high solid latex is desirable in variety of commercial applications.

Surprisingly it has now been found that a fluoropolymer can be made by a process using non-fluorinated, non-ionic emulsifiers containing segments of polyethylene glycol and/or polypropylene glycol having varieties of different terminal groups and functions. The fluoropolymer dispersions produced have good latex stability and shelf-life, and are coagulum and adhesion free.

SUMMARY OF THE INVENTION

The invention describes an aqueous fluoropolymer composition comprising

-   -   a) a fluoropolymer containing at least 50 mole percent of         fluoromonomer units; and     -   b) from 100 ppm to 2 percent, based on the weight of the         fluoropolymer solids, of one or more emulsifier(s) having         polyethylene glycol and/or polypropylene glycol segments with         repeating units of from 2 to 100.

The invention also describes a process for preparing a fluoropolymer comprising polymerizing at least one fluoromonomer in an aqueous medium comprising at least one emulsifier consisting of a non-fluorinated, non-ionic emulsifier containing polyethylene glycol and/or polypropylene glycol segments with repeating units between 2 to 200.

DETAILED DESCRIPTION OF THE INVENTION

The term “fluoromonomer” as used according to the invention means a fluorinated and olefinically unsaturated monomer capable of undergoing free radical polymerization reaction. Suitable exemplary fluoromonomers for use according to the invention include, but are not limited to, vinylidene fluoride, vinyl fluoride, trifluoroethylene, tetrafluoroethylene (TFE), and hexafluoropropylene (HFP) and their respected copolymers. The term “fluoropolymer” refers to polymers and copolymers (including polymers having two or more different monomers, including for example terpolymers) containing at least 50 mole percent of fluoromonomer units.

The term “vinylidene fluoride polymer” used herein includes both normally solid, high molecular weight homopolymers and copolymers within its meaning. Such copolymers include those containing at least 50 mole percent of vinylidene fluoride copolymerized with at least one comonomer selected from the group consisting of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, pentafluoropropene, perfluoromethyl vinyl ether, perfluoropropyl vinyl ether and any other monomer that would readily copolymerize with vinylidene fluoride. Particularly preferred are copolymers composed of from at least about 70 and up to 99 mole percent vinylidene fluoride, and correspondingly from 1 to 30 percent tetrafluoroethylene, such as disclosed in British Patent No. 827,308; and about 70 to 99 percent vinylidene fluoride and 1 to 30 percent hexafluoropropene (see for example U.S. Pat. No. 3,178,399); and about 70 to 99 mole percent vinylidene fluoride and 1 to 30 mole percent trifluoroethylene. Terpolymers of vinylidene fluoride, hexafluoropropene and tetrafluoroethylene such as described in U.S. Pat. No. 2,968,649 and terpolymers of vinylidene fluoride, trifluoroethylene and tetrafluoroethylene are also representatives of the class of vinylidene fluoride copolymers which can be prepared by the process embodied herein.

Emulsifiers suitable for use in this invention are non-fluorinated non-ionic emulsifiers containing segments of polyethylene glycol (PEG), polypropylene glycol (PPG) or a combination thereof, with repeating units between 2 to 200, preferably between 3 to 100, and more preferably 5 to 50. The glycol-based emulsifiers used in this invention include, but are not limited to, polyethylene glycol acrylate (PEGA), polyethylene glycol (PEG), polyethylene glycol phenol oxide (Triton X-100), polypropylene glycol acrylate (PPGA), and polypropylene glycol (PPG).

The emulsifier may contain the same or different terminal groups on each end, such as hydroxyl, carboxylate, benzoate, sulfonic, phosphonic, acrylate, methacrylate, ether, hydrocarbon, phenol, functionalized phenol, ester, fatty ester, and the like. The terminal group can contain halogen atoms like F, Cl, Br and I, and also other groups or functions such as amine, amid, cycle hydrocarbon, and others. For example, polyethylene glycol acrylate with Mn 375, polyethylene glycol with Mn 570, and polyethylene glycol with phenol oxide end group and many other example can be used in this invention to produce stable fluoropolymer latex with small particle size.

The chemical structure of the emulsifier of this invention could be altered so that PEG and/or PPG would not be the main backbone but the essential properties such as water solubility, chain transfer activities, and protective behaviors remains the same.

The emulsifier is used at a level of from 100 ppm to 2 percent, 100 ppm to 1 percent and 100 ppm to ½ percent, based on the total polymer solids of the fluoropolymer formed.

In the polymerization process, the emulsifier of this invention could be added all upfront prior to polymerization, fed continuously during the polymerization, fed partly before and then during polymerization, or fed after polymerization started and progressed for a while.

The manner of practicing the invention will now be generally described with respect to a specific embodiment thereof, namely polyvinylidene fluoride based polymer prepared in aqueous emulsion polymerization using non-fluorinated non-ionic emulsifier as the principle emulsifier. Although the process of the invention has been generally illustrated with respect to the polymerization of vinylidene fluoride based polymers, one of skill in the art will recognize that analogous polymerization techniques can be applied to the preparation of homopolymers and copolymers of fluorinated monomers in general, and more specific in VDF, TFE, and/or CTFE with co-reactive monomers fluorinated or non-fluorinated such as hexafluoropropylene, perfluorovinyl ether, propane, vinyl acetate, and the like.

The reactor temperature is raised to the desired polymerization temperature, the predetermined amount of either vinylidene fluoride alone or a mixture of monomers such as vinylidene fluoride and hexafluoropropylene are fed to the reactor. The temperature of the reaction can vary depending on the characteristics of the initiator used, but is typically from about 30° to 140° C., preferably from about 50° to 130° C. Once the pressure in the reactor has reached the desired level, an initiator solution, made of either potassium persulfate, ammonium persulfate, or an emulsion of one or more organic peroxides such as propyl peroxidicarbonate, or dibutylperoxide in water, is charged to start the polymerization reaction. The polymerization pressure may vary, but typically will be within the range of about 20 to 50 atmospheres. Following the initiation of the reaction, the vinylidene fluoride or vinylidene/hexafluoropropylene mixture is continuously fed along with additional initiator to maintain the desired pressure. Once the desired amount of polymer has been reached in the reactor, the monomer feed(s) will be stopped, but initiator feed is continued to consume residual monomer(s). In order to avoid compositional drifts in case of copolymers, after reactor pressure drops to a given level, a shot of vinylidene fluoride is added to bring the vinylidene fluoride concentration up. This step may be repeated more than one time depending on the hexafluoropropylene concentration in the reactor. When the reactor pressure is low enough, about 300 psig, the initiator charge is stopped and after a delay time the reactor is cooled. The unreacted monomer(s) are vented and the latex is recovered from the reactor. The polymer may then be isolated from the latex by standard methods, such as acid coagulation, freeze thaw or shear coagulation.

A paraffin antifoulant is an optional additive, and any long-chain, saturated, hydrocarbon wax or oil may be used for this purpose. Reactor loadings of the paraffin typically are from 0.01 percent to 0.3 percent by weight on the total monomer weight used.

A chain transfer agent may be added all at once at the beginning of the reaction, or it may be added in portions, or continuously throughout the course of the reaction. The amount of chain transfer agent added and its mode of addition depends on the desired molecular weight characteristics, but is normally used in an amount of from about 0.5 percent to about 5 percent based on total monomer weight used, preferably from about 0.5 percent to about 2 percent.

When copolymerization of vinylidene fluoride and hexafluoropropylene are performed, or copolymerization of any two coreactive fluorinated monomers having differing reaction rates, the initial monomer charge ratio and the incremental monomer feed ratio during polymerization can be adjusted according to apparent reactivity ratios to avoid compositional drift in the final copolymer product.

The reaction can be started and maintained by the addition of any suitable initiator known for the polymerization of fluorinated monomers including inorganic peroxides, “redox” combinations of oxidizing and reducing agents, and organic peroxides. Examples of typical inorganic peroxides are the ammonium or alkali metal salts of persulfates, which have useful activity in the 65° C. to 105° C. temperature range. “Redox” systems can operate at even lower temperatures and examples include combinations of oxidants such as hydrogen peroxide, t-butyl hydroperoxide, cumene hydroperoxide, or persulfate, and reductants such as reduced metal salts, iron (II) salts being a particular example, optionally combined with activators such as sodium formaldehyde sulfoxylate, metabisulfite, or ascorbic acid. Among the organic peroxides which can be used for the polymerization are the classes of dialkyl peroxides, diacyl-peroxides, peroxyesters, and peroxydicarbonates. Exemplary of dialkyl peroxides is di-t-butyl peroxide, of peroxyesters are t-butyl peroxypivalate and t-amyl peroxypivalate, and of peroxydicarbonate, and di(n-propyl) peroxydicarbonate, diisopropyl peroxydicarbonate, di(sec-butyl) peroxydicarbonate, and di(2-ethylhexyl) peroxydicarbonate. The use of diisopropyl peroxydicarbonate for vinylidene fluoride polymerization and copolymerization with other fluorinated monomers is taught in U.S. Pat. No. 3,475,396 and its use in making vinylidene fluoride/hexafluoropropylene copolymers is further illustrated in U.S. Pat. No. 4,360,652. The use of di(n-propyl) peroxydicarbonate in vinylidene fluoride polymerizations is described in the Published Unexamined Application (Kokai) JP 58065711. The quantity of an initiator required for a polymerization is related to its activity and the temperature used for the polymerization. The total amount of initiator used is generally between 0.05% to 2.5% by weight on the total monomer weight used. Typically, sufficient initiator is added at the beginning to start the reaction and then additional initiator may be optionally added to maintain the polymerization at a convenient rate. The initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen. As a particular example, peroxydicarbonates are conveniently added in the form of an aqueous emulsion.

While the invention is generally practiced with the PEG and/or PPG emulsifier as the sole emulsifiers, co-emulsifiers or co-surfactants could also be present in the invention, including fluorinated or partially fluorinated emulsifiers.

The process of present invention is easy, convenient, cost effective, and more importantly is coagulum and adhesion free. The resulting polymer dispersions have good latex stability and shelf-life, and a good quality of film formation. Additionally, the particle size of dispersion could be small (<100 nm) which in turn would be advantageous for many direct applications of fluoropolymer in a latex form. Furthermore, the fluoropolymer produced with the process of this invention, has a higher purity, with less extractable ions and less low molecular weight polymers.

The following examples further illustrate the best mode contemplated by the inventors for the practice of their invention and should be considered as illustrative and not in limitation thereof.

EXAMPLES

The glycol-based emulsifiers used in this example include polyethylene glycol acrylate (PEGA), polyethylene glycol (PEG), and polyethylene glycol phenol oxide (Triton X-100), polypropylene glycol acrylate (PPGA), and polypropylene glycol (PPG). Inspection of results in the following table indicates that a with a low loading of the emulsifiers, emulsions of fluoropolymers having particle sizes of approximately 100 nm were produced. The solid level of these novel emulsions were as high as 42%.

To a 1.7 liter agitated-autoclave reactor was added one liter of DI-water along with the reported amount of 10% aqueous solution of emulsifier (as shown in Table 1). The mixture was purged with argon and then heated to desired temperature. The reactor was then charged with VF2/HFP to reach pressure of 4510 kPa. A continuous feed of the 1% aqueous initiator solution was added to the reaction and the pressure was maintained at 4480 kPa by adding as needed VF2/HFP. After the pre-determined amount of VF2 in the reactor was reached, addition of monomers were stopped and only initiator addition of initiator was continued till the pressure in the reactor was dropped to 300 psi. After cooling to room temperature, the reactor was emptied. Gravimetric solids and particle size measurements of the latex were conducted.

TABLE 1 surfactant Particle Surfactant solution, Initiator⁽¹⁾ VDF⁽²⁾ Solids size type g ml ml HFP⁽³⁾ % (nm) PEGA 7.5 186 473 193 37 108 PEGA 7.5 154 449 114 29 116 PEGA 4 100 541 160 37 143 X-100 7.5 195 453 202 35  79 PEG (200) 7.6 64 451 194 35 232 PEG (300) 7.5 109 450 191 36 235 PEG 7.5 122 360 217 42 — PEG (570) 7.5 127 450 200 35 215 PPG (450) 7.5 123 450 198 36 — PPGA 7.5 141 449 212 36 100 PPGA 3 99 450 202 35 150 PPGA 5.11 78 450 106 33 122 PPGA 5 59 500 102 32 123 PPGA 5 58 549 0 27 — PPGA 6 81 650 0 32 — ⁽¹⁾Initiator solution was made of 1% potassium persulfate and 1% sodium acetate ⁽²⁾Density of VDF at the feeding condition is 0.83 g/ml ⁽³⁾Density of VDF at the feeding condition is 1.35 g/ml 

1. A process for preparing a fluoropolymer comprising polymerizing a monomer mixture consisting of 70 to 100 mole percent of vinylidene fluoride and 0 to 30 mole percent of one or more other fluoromonomers in an aqueous medium comprising from 100 ppm to 2 percent, based on the weight of the fluoropolymer solids of at least one emulsifier consisting of a non-fluorinated, non-ionic emulsifier containing polyethylene glycol and/or polypropylene glycol segments with repeating units between 2 to 200, and using as the initiator one or more initiators selected from the group consisting of ammonium persulfate, alkali metal salts of persulfates, dialkyl peroxides, diacylperoxides, peroxyesters, and peroxydicarbonates.
 2. The process of claim 1 wherein said emulsifier(s) is present in the process at from 100 ppm to 1 percent, based on the weight of the fluoropolymer solids, of one or more emulsifiers having polyethylene glycol and/or polypropylene glycol segments.
 3. The process of claim 1 wherein said emulsifier(s) is present in the process at from 100 ppm to ½ percent, based on the weight of the fluoropolymer solids, of one or more emulsifiers having polyethylene glycol and/or polypropylene glycol segments.
 4. The process of claim 1 wherein said emulsifiers are selected from the group consisting of polyethylene glycol acrylate, polyethylene glycol, polyethylene glycol phenol oxide, polypropylene glycol acrylate, and polypropylene glycol.
 5. The process of claim 1 wherein said emulsifier(s) contains polyethylene glycol and/or polypropylene glycol segments with repeating units of from 3 to
 100. 6. The process of claim 1, wherein said polymerization is initiated with initiator that is selected from the group consisting of ammonium or alkali metal salts of persulfates. 